Project Summary Retinoblastoma and osteosarcoma are the third and eight most common forms of childhood cancers, respectively. Children with hereditary retinoblastoma -germline mutation in the RB1 gene- are predisposed to osteosarcoma later in their life. Despite recent progress in clinical outcomes in retinoblastoma and osteosarcoma, enucleation remains a frequent treatment for retinoblastoma and the survival rate for osteosarcoma is just over 50%, underscoring the need to identify molecular mechanisms responsible for disease progression and to develop more effective drugs. Previous finding from our laboratory suggest that aE2Fs regulate recruitment of the epigenetic machinery that is required for tumor formation in the absence of the Rb family and that epigenetic deregulation of genes in the absence of Rb is sufficient for retinoblastoma formation. However, while functional loss of RB1 is enough for retinoblastoma formation, osteosarcoma requires inactivation of an additional tumor suppressor, TP53. Through the study of the epigenetic landscape of retinoblastoma and osteosarcoma, in this research proposal we hope to determine why some cell types are more susceptible to tumor formation than other cell types, in particular following RB1 inactivation. We aim to identify direct transcriptional targets of Rb epigenetically regulated by activator E2Fs in retinoblastoma and elucidate the role of epigenetics and genome instability in osteosarcomagenesis and how Rb participates in this process. The use integrative analyses of the changes in chromatin organization and gene expression that occur during tumorigenesis will help us identify potential novel therapeutic targets for anticancer treatment.